Scientific knowledge is particularly rigorously tested knowledge that aims to distinguish itself from mere opinion. This claim is based on the methodical examination of scientific assertions against experience. A key part of the philosophy of science is the theory of methods, which deals with which testing procedures are accepted in science and how they relate to the scientific claim to knowledge. Therefore, the focus of this introduction is on the procedures for ensuring validity in science throughout its historical development. Important challenges for the theory of methods arise, among other things, from the fact that science must pursue its goal of knowledge within the tension between societal expectations of utility and value guidelines.